1. Technical Field
The present invention relates to a kind of biodegradable polyester and its preparation method, which belongs to the field of biodegradable co-polyester product technology.
2. Description of Related Art
Biodegradable polymer is a kind of polymer material that may decompose into carbon dioxide and water under appropriate environmental conditions over a certain period. The degradation process is normally divided into two steps: first, the molecular weight of macro molecules decreases by hydrolysis and light/oxygen degradation, then it is further consumed by microorganisms. The microorganisms could be bacteria, fungi, microzyme and algae. A kind of testing method for biodegradability is provided in the international standard ISO14855, which is a relatively authoritative testing method in the testing of biodegradability for plastic materials. Various countries and regions define their own plastics degradation tests and testing standards according to the testing conditions and results, including the EN13432 testing standard developed by EU, the ASTM D6400 of U.S.A, the GB/T 19277 of China, etc.
Polyhydroxyalkanoate (PHA), for example, polylactic acid (PLA), polyhydroxybutyrate (PHB), polycaprolactone (PCL), poly-hydroxybutyrate-valerate (PHBV), has a history over 30 years, all of which, except PCL, could be obtained by biosynthesis with biodegradability (M. Kunioka et al, Appl. Microbiol. Biotechnol., 30, 569, 1989). It is also pointed out by some reports that the polyester obtained from condensation polymerization of aliphatic dibasic acid (or ester) and dibasic alcohol also has biodegradability (written by J. M. Sharpley et al, “Application Science”, 1976, p. 775). All the polyester materials obtained completely from aliphatic dibasic alcohol and dibasic acid have relatively low melting points and vitrification temperatures, and there are defects in their application.
Aromatic polyester, for example, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), etc., is the plastic material with very wide application; however, such materials have no degradability (Kirt-Othmer Encyclopedia of Chemical Technology, Suppl. Vol., Wiley-Interscience, New York, 1984, p. 626-668). In early 1980's, there was relevant report pointing out the viewpoint of Y. Tokia and T. Suzuki in some article (Nature, 270, 76-78, 1977; Journal of Applied Polymer Science, 26, 441-448, 1981) that the aliphatic polyester obtained from the condensation polymerization of succinic acid and aliphatic dibasic alcohol could be degraded by enzyme; however, the polyester formed by aromatic dibasic acid and aliphatic dibasic alcohol, for example, PBT and PET, could not be degraded by enzyme; the block copolyester resulting form PCL and PBT could be degraded by enzyme.
In Patent WO 92/13019, a kind of polyester copolymer formed by aromatic dibasic acid and aliphatic dibasic alcohol has biodegradability. The structure of such copolyester requires that the dibasic alcohol segment with the minimum molar ratio of 85% in the polyester contains one terephthalic acid segment. To improve the material hydrophilic property and reduce crystallization, it is proposed by this patent to introduce metal salt of dimethyl isophthalate-5-sulfonic acid with the molar ratio of 2.5% or dibasic alcohol unit containing ether structure of chain scission into the copolymer structure. However, the patent has no microorganism degradation results of the material; only the water-boiling test was carried out, and the mechanical property of the material is not satisfactory.
It is revealed by U.S. Pat. No. 5,292,783 and U.S. Pat. No. 5,446,079 the block and linear irregular copolyester obtained by condensation polymerization with aliphatic dibasic acid and aromatic dibasic acid as repetitive units. Such material has the biodegradability. Among others, the dibasic carboxylic acid consists of 5-65% (molar ratio) aliphatic dibasic acid and 35-95% (molar ratio) aromatic dibasic acid, and the dibasic alcohol is aliphatic dibasic alcohol. However, since the materials have relatively low melt viscosity and melt strength, they cannot be applied in the intrusion processing field, for example, it is relatively difficult to use the material for film blowing, foaming and flow casting.
U.S. Pat. No. 5,661,193 reveals a kind of aliphatic acid—aromatic acid copolyester with a branching and irregular structure, which has the biodegradability. It is used to produce foam material. The polyester consists of 30-95% (molar ratio) polycondensate units of aliphatic dibasic acid, 5-70% (molar ratio) polycondensate units of aromatic dibasic acid, and the dibasic alcohol units in the polycondensate units are polycondensate units of aliphatic dibasic alcohol. The content of branching agent is 0.01-10% of the weight of dibasic acid used for polymerization. The branching agent revealed in the patent is multi-carboxyl aliphatic acid and (or) anhydride, multi-carboxyl aromatic acid and (or) anhydride, multi-hydroxyl aliphatic alcohol and hydroxyl isocyanurate.
Patent EP A565235 proposes a kind of aliphatic copolyester containing amino formyl structural units (—NH—C(O)O—). The basic units for the copolyester are succinic acid and aliphatic dibasic alcohol, which have the biodegradability. To change the defect of relatively low molecular weight resulting from condensation polymerization of pure aliphatic acid and alcohol, diisocyanate reaction units are introduced into the reaction. However, diisocyanate reaction unit is easy to generate gelling point in the reaction, it is relatively difficult to control the reaction, and the appearance of gel will impact the performance of the material.
U.S. Pat. No. 6,018,004 also reveals several kinds of polyester materials, which also have biodegradability. Among others, the polycondensate units of dibasic acid in a kind of biodegradable polyester consist of 35-95% (molar ratio) polycondensate units of aliphatic dibasic acid, 5-65% (molar ratio) polycondensate units of phthalandione and 0-5% (molar ratio) sulfonate, in which the polycondensate units of dibasic alcohol are alkyl dibasic alcohol and cycloalkyl dibasic alcohol. The above-mentioned composition of the polyester could form another kind of copolyester with biodiagradability with the following structure: chemical substances containing hydroxyl and carboxyl with over three functional groups at the molar ratio of 0.01-5% (take the total mole number for the polycondensate units of dibasic acid as 100). The patent has relatively detailed definition for such substances, including aromatic polybasic acid, aliphatic polybasic acid, aliphatic polybasic alcohol, aromatic hydroxyl acid, etc; the structure could also be diisocyanate-type chemical substances with the weight percentage of 0.1-5%. The patent also defines such substances in details, including aromatic diisocyanate and aliphatic diisocyanate.
U.S. Pat. No. 6,120,895 reveals a kind of polyester material with biodegradability. The polyester material consists of two parts, Component A with the molar ratio of 95-99.9% and Component B with the molar ratio of 0.01-5%: Component A consists of the chemical substances including 20-95% (molar ratio) aliphatic dibasic acid (or its ester), 5-80% (molar ratio) aromatic dibasic acid (or its ester) and dibasic hydroxyl and amido alcohols; Component B consists of single-cluster or multi-cluster isocyanate chemical substances. The preparation method has both the characteristics of polycondensation reactions and solidification reaction for polyester. The dibasic alcohol used in Component A is aliphatic dibasic alcohol or polyether dibasic alcohol. Although the introduction of isocyanurate may raise the heat-resistant property of the material, the isocyanurate defined in the patent makes reaction control very difficult with many gelling points.
Up to now, the biodegradable polyester resin materials could not meet the preparation requirements, especially in terms of the requirements on the performance and production requirements of membrane material. Even with chain extension or branching treatment during synthesis, the aliphatic polyester still has a relatively low melting point, insufficient heat resistance and picking defect during processing. The linear aliphatic/aromatic copolyester with diisocyanate chain extension treatment is easier for the processing of membrane materials compared with the polyester materials without chain extension treatment, but the formed gel particles will interfere with the processing of membrane materials, especially when the cycle period extends, and the mechanic properties is relatively poor.